X-ray imaging is useful in dentistry because it reveals information about the inside of a tooth. This includes cavities or voids in the dental tissue, and also includes demineralized areas where the mineral content of the tissue has been reduced and the tissue becomes porous, typically as a result of acid in contact with the tooth. X-ray imaging works because cavitated or demineralized areas are more transparent to x-rays than surrounding tissues, and therefore transmit a higher intensity of the x-rays emanating from a source, creating a greater exposure on photographic film or on an electronic imaging device than the exposure created by radiation coming along nearby paths that do not intercept cavitated or demineralized zones. Since x-rays travel in nearly straight lines through the tooth, imaging is generally accomplished, not by focusing with a lens, but by using a very small source for the x-rays, so that a graded shadow is produced on the film or electronic detector. The size of the source will determine the degree to which the image is sharp or blurry.
X-ray imaging has several disadvantages. X-rays ionize molecules in living tissue and are therefore dangerous. The contrast in x-ray images of small caries is poor, because a small carious volume with increased transparency or reduced attenuation only makes a small fractional change in the intensity of transmitted radiation even if the carious region is perfectly transparent, as when it is void of matter. Interproximal caries (on the sides of teeth) can often be seen despite this disadvantage because x-rays from these caries come through relatively little matter near the edges of the tooth. Occlusal caries (on or just below the biting surfaces of molars) often cannot be seen at all, because these biting surfaces are generally broad and flat, so that the x-rays are transmitted through a large amount of matter which is quite opaque, and because the fractional change due to caries is small. It would be valuable to observe the lateral extent of occlusal decay by looking vertically through the biting surfaces of molars, but X-rays can generally only be used through the sides of teeth (traversing horizontally when the patient's head is upright). Even if x-rays could be used vertically, the opacity due to the long distance through the dental tissue would probably make this geometry ineffective for viewing the lateral extent of occlusal decay.
In addition to the detection of caries, dentists need to detect cracks in teeth, particularly when replacing an old filling and deciding whether to apply another inlay (which is acceptable if there is no crack but problematic if there is one) or to apply a crown (which is indicated if there is a crack). But x-ray techniques are not reliable for detecting cracks. A crack may show if it happens to be aligned with the beam, but will not show at all if it is not aligned.
Attempts have been made to use visible light for detecting dental anomalies. These techniques avoid ionizing radiation. Unlike x-rays, however, visible light does not travel straight through a typical thickness of dental enamel or dentin, but is scattered randomly in all directions. This makes teeth appear milky white, and prevents detection of deep anomalies because the scattering seriously blurs the light. A commercial product that records digital images made with light in the visible wavelengths has been shown to be quite ineffective for detecting and characterizing interproximal lesions in comparison to x-ray. See, for example, Young and Featherstone, “Comparing digital imaging fiber-optic trans-illumination, F-speed radiographic film, and polarized light microscopy,” in Early Detection of Dental Caries. III: Proceedings of the 6th Annual Indiana Conference, G. K. Stookey, ed. (2003). This product can only detect anomalies that are on or very near the surface of the tooth, and cannot determine their depth or whether they have penetrated through the enamel and into the dentin, which is important for decisions on therapy.
Imaging with infrared light can reduce the problems with x-rays and visible-light imaging because dental enamel is substantially more transparent to infrared light than to visible light. But methods that use infrared cameras present additional problems. This disclosure describes some of those problems and presents methods and apparatus that mitigate them in ways that are suitable for practical, widespread commercial use of infrared dental imaging.